The present invention relates to propylene-based copolymers.
Particularly, it relates to propylene-based copolymers that have haze appropriate for matte film and are superior in appearance and rigidity, and also relates to films made of the same. In this specification, the expression “a film is superior in appearance” means primarily that the film has no or almost no fisheyes as large as they can be visually recognized.
Polypropylene is superior in rigidity, heat resistance and suitability for packaging and therefore is used widely in the field of materials for packaging such as food packaging and fiber packaging. In recent years, luxurious matte films comprising polypropylene as a major component have been awaited; for example, JP 56-38339 A discloses that a film having been improved in haze, which is a measure indicating the degree of translucency, can be obtained from a polypropylene resin composition in which a crystalline propylene-ethylene block copolymer has been mixed with such an organic peroxide that the temperature at which a half life of 10 hours is obtained is 60° C. or higher, JP 2-69549 A discloses that a product having been improved in matting effect can be obtained from a polypropylene-based resin composition to be obtained by a production method comprising heat-treating a mixture of a block copolymer containing a propylene homopolymer portion and an ethylene-propylene random copolymer portion which are obtained through polymerization on the same catalyst, polyethylene, a cross-linking aid, and radical generator, JP 2003-213068 A discloses that a polypropylene-based resin composition containing a propylene-based copolymer composed of a polymer component to be obtained by polymerizing a monomer component mainly comprising propylene and a propylene-ethylene copolymer component, and a propylene-based polymer improves the appearance and the moist feeling of a matte film.
However, further improvement in the appearance, particularly the balance between the haze and the quantity of fisheyes, of matte films has been desired.